Anti-Brake lock systems for vehicles customarily include sensors which sense the course of wheel speed during operation of the vehicle. An evaluation circuit is coupled to receive the signals from the wheel speed sensors to generate brake pressure control signals to control hydraulic or pneumatic pressure (or vacuum). The vehicle brakes or, rather, valves controlling admission of brake fluid to the brakes, are controlled by the signals from the evaluation circuit. The braking pressure at the respective wheel brakes is varied, permitting, for example, increase in braking pressure if the brake to be effective, drop in braking pressure if wheel slip or skidding is sensed, or an impending slip or skid is sensed. The braking pressure may also be maintained constant, for example by closing both a pressure inlet as well as a pressure outlet valve. Selectively opening the inlet and outlet valves, respectively, increases and lowers braking pressure.
The evaluation circuit, after an initial increase in braking pressure, may sense that the braked wheel has a tendency to skid and control the respective brake valve to lower the braking pressure. When the tendency to skid or the actual skid has terminated, as sensed, for example, by the wheel speed sensor and signal processing involving evaluation of the rate of change of wheel speed, or the rate of acceleration/deceleration of wheel speed, the braking pressure can be increased again. The level of increase of the braking pressure in a cycle, which may be denoted .DELTA.p.sub.2, can be made dependent on the extent of prior drop in braking pressure, denoted .DELTA.p.sub.1. Further braking pressure rise and pressure braking drop cycles may follow.
A vehicle anti-brake lock system (ABS) of the type described is shown in German Patent Publication Document DE-OS No. 24 60 904 (U.S. Pat. No. 4,054,328), particularly FIG. 2. As described, the first brake pressure rise following a brake pressure drop is made dependent on the pressure rise of one or more preceding control cycles. The very first control cycle, however, for which there is no preceding pressure rise, may be controlled, for example, by storing a prior brake pressure drop, and then consider such prior stored brake pressure drop as the factor on which a first brake pressure rise is considered.